1. Field of the Invention
The present invention relates to a process and an apparatus for the recovery of liquid phases from three phase emulsions. More specifically the invention relates to a process and an apparatus for the recovery of organic extractant and phosphoric acid from stable emulsions formed during the extraction of uranium from wet-process phosphoric acid.
2. Description of the Background Art
Wet process phosphoric acid is produced by reacting phosphate rock with sulfuric acid. The major reaction products include phosphoric acid and calcium sulfate (gypsum) which are separated by filtration. The 30 percent P.sub.2 O.sub.5 phosphoric acid stream from the filtration operation contains other constituents, including uranium, which can be recovered via solvent extraction at this point in the process. The uranium-free 30 percent P.sub.2 O.sub.5 phosphoric acid is then concentrated by evaporation to about 54 percent which is the concentration at which the merchant grade acid is sold. The uranium extracted from the 30 percent P.sub.2 O.sub.5 phosphoric acid stream is sold as a by-product.
Depending on the source, phosphate rock contains various natural organic impurities. Other organic impurities can be imparted to the phosphate rock during pretreatment operations. Some of the organic impurities pass through the gypsum filter cake and report to the 30 percent P.sub.2 O.sub.5 phosphoric acid stream described above. These impurities, commonly referred to as humates or humic acids, are responsible for creating and stabilizing emulsions formed when phosphoric acid is mixed with solvent in the uranium solvent extraction process. The emulsions, known in the industry as "cruds", not only create processing problems but also contribute substantially to the uranium production costs by increasing solvent losses.
The cruds are three-phase emulsions consisting of a "light" or light-density liquid phase, a "heavy" or heavy-density liquid phase and a solid phase. The light-density liquid phase, which can be referred to as the "solvent", the "organic", or the "organic extractant", is composed of active ingredients which are dissolved in an inert diluent. In the case of uranium extraction from phosphoric acid, the active ingredients are di-2-ethyl-hexyl phosphoric acid, which is commonly referred to as DEHPA, and tri-octyl-phosphine-oxide, which is commonly referred to as TOPO. These active ingredients are dissolved in the inert diluent which is, typically, kerosene. This light-density phase typically has a density of about 0.83. The high density liquid phase, which is normally referred to as the "aqueous" or acid phase, is usually an acid or base leach solution. In the case of uranium extraction from phosphoric acid the aqueous component of crud is phosphoric acid. This heavy-density phase typically has a density of about 1.3. Both of these liquid phases of crud are of economic value to the processing facility and it is, therefore, desirable to recover these liquids. The solid phase is composed of organic and inorganic solids. The organic solids include the humates discussed above which typically display an effective density, when wet, of that between the two liquid phases. Once separated from the liquid phases, washed free of any solvent, and removed from any filter aid, crud solids are tar-like and composed of about 80 percent carbonaceous matter with the remaining portion including P.sub.2 O.sub.5, residual moisture, and inorganic solids.
Crud solids in the emulsion have small particle sizes and a gelatinous characteristic. These characteristics of crud solids cause filtration rates of crud to be too low to effectively utilize either pressure or vacuum filtration of large quantities of these solids. The relatively low density of the solid phase of crud does not allow a continuous decanter centrifuge to effectively process crud. Such centrifuges can contiuously discharge heavy, dense solids by throwing such solids against the wall of the centrifuge chamber. Crud solids are not dense enough for such a procedure. The characteristics of crud solids prevent the formation of distinct solid-liquid interfaces during centrifugation in such machines.
Other types of continuous centrifuges such as nozzle or disk centrifuges or those described in U.S. Pat. No. 4,424,195 to Korchnak et al. cannot achieve three phase separation of crud. The separation products from such centrifuges consist of a clear overflow of solvent and an underflow of a mixture of acid, solvent, and solids. Since the solids can only be effectively discharged in slurry form, solvent recovery is not maximized and the acid is not discharged as a clear liquid from these centrifuges. Also, the use of these centrifuges in a crud processing system provides poor results because excessive wear rapidly occurs in the centrifuges. This rapid wear occurs because of the high speeds of operation of these centrifuges. These centrifuges can operate at speeds from 4000 to 6000 revolutions per minute (RPM). Pluggage by the tar-like, carbonaceous crud solids occurs in the ports and chambers of these centrifuges.
Solid bowl or imperforate basket centrifuges equipped with internal skimmer tubes have been used successfully to make liquid-solid separation in two phase systems. In such applications, where the solids are more dense than the liquid, the solids are thrown against the centrifuge wall, while a pool of clarified liquid accumulates within the centrifuge. If the liquid is not of value, it is usually allowed to accumulate to a sufficient depth to overflow the centrifuge basket lip. This method of continuous liquid discharge results in splashing and some of the liquid is lost through the bottom solids discharge chute of the centrifuge basket. If the liquid is of value it can be removed via an internal skimmer tube. Such a machine can be used to achieve the desired separation with three phase emulsions such as cruds. However, commercial success is limited by the ability to continuously remove the heavy phase liquid once the separation has been achieved. Also, the solids must be discharged in slurry form thereby removing excessive amounts of acid and solvent.
Chemical treatment methods, as described in U.S. Pat. No. 4,190,633 entitled "Crud Handling Circuit" to Smith et al. commonly assigned with this invention, offer alternative methods to mechanical separation with centrifuges. In some plant facilities, however, it is not feasible to use chemical treatment methods because of difficulties in disposing of the additional chemical residue.
The industry has recognized the problems associated with recovering the solvent and acid from crud and has, therefore, sought to minimize the problem by removing crud-forming matter from the phosphoric acid prior to any uranium extraction process. Several methods of removing crud-forming matter from phosphoric acid have been developed. U.S. Pat. No. 4,087,512 to Reese et al. discloses an attempt to minimize the formation of stable emulsions, that are caused by humic acids in wet process phosphoric acid feed, by a pretreatment with an inexpensive hydrocarbon. Such hydrocarbons can include kerosene, gasoline, benzene, and toluene. In this process the acid and hydrocarbon are mixed and then transferred to a three phase separator where, after a few minutes, free acid and solvent separate leaving behind the stable emulsion which contains the hydrocarbon, acid, and crud forming solids.
The Reese et al. process discussed above and similar processes can, in varying degrees, reduce solvent losses and other downstream problems associated with crud formation. However, even the most effective of the existing commercial processes cannot totally eliminate crud generation. Thus, the need for an efficient solvent recovery process exists within the industry. Such a process can have the benefit of precluding the need for expensive acid clean-up systems. The industry, therefore, requires a process by which solvent and acid can be efficiently recovered from crud formed during the recovery of uranium from phosphoric acid, and equipment to perform this process.